HIGH-RESOLUTION SEQUENCE STRATIGRAPHY AND EVENT BED CORRELATIONS OF THE UPPER ORDOVICIAN BLACK RIVER AND TRENTON GROUPS IN CENTRAL PENNSYLVANIA: UNRAVELING (ADDING TO?) A CENTURY OF CONTROVERSY

Despite their proximity to type sections of the Black River (BR) and Trenton (TR) Groups in NY, Mohawkian strata of central PA have been difficult for establishing precise regional correlations. These difficulties are promulgated by the general facies change to deeper water carbonates and the difficult identification of key lithologic units and time-rock intervals of the type region (i.e. Rocklandian, Kirkfieldian, Shermanian). In spite of this problem, previous researchers established correlations on the biostratigraphic occurrence of “key” TR taxa: the occurrence of Maclurites, Hesperorthis, Doleroides within the Centre Hall (L Nealmont Fm) and Echinosphaerites sp within the Rodman (U Nealmont): Rockland-Kirkfield of NY; the occurrence of Cryptolithus in the overlying Salona Fm: Sugar River of NY (E Shermanian). Early qualitative K-bentonite assessments were utilized to help “firm-up” these time-rock classifications. However, as many of these taxa are now known to be facies controlled and fairly long-ranging within the Mohawkian, these correlations have been challenged.

Building on previous and ongoing research, studies of the BR to TR interval in NY/Ontario and KY/OH have been successful in establishing a framework of depositional sequences constrained by K-bentonites, disturbed horizons, epibole horizons, hardgrounds, isotopic shifts, and biostratigraphic correlations. This framework provides many opportunities to establish litho, bio, and chronostratigraphic position of these Mohawkian sequences in the intervening region of central PA. We recognize 6 large-scale depositional sequences (M1-M6 of Holland & Patzkowsky) and 6 smaller-scale sequences (M5: A-C, M6: A-C) within the BR to TR interval of PA. Our studies in PA have adjusted previous time-rock correlations. It is suggested that the base of the M5A sequence (base Watertown of NY), be placed at the SB at the base of the Centre Hall and the M5A MFS be placed at the top of the Rodman (top Selby of NY). This correlation is consistent with conodont correlations, and recent carbon isotopic studies. This correlation, however, is at odds with earlier quantitative K-bentonite correlations which suggest the placement of the M4-M5A SB within the Salona. More recent chemical fingerprinting methods are now being employed to test these correlations.